It seems as though if you can get an erection, you can attain ejaculation, even if you wouldn't feel the sensation. But if you are talking about some kind of nerve damage that makes it impossible to get an erection, then ejaculation would not be possible either.
If " . . . you have zero sensation below your chest", you would have to somehow correct your lack of feeling, if possible. Otherwise, move on to other attainable goals, since an orgasm is not possible in this situation.